1. Field of the Invention
The present invention relates to switching power supply apparatuses and, more particularly, to a resonance-type switching power supply apparatus using resonance to perform a power conversion operation.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 9-308243 and Japanese Unexamined Patent Application Publication No. 11-332232 each disclose a resonance-type switching power supply apparatus using resonance to perform a power conversion operation.
In the switching power supply apparatuses adopting current resonance converters, which use resonance for their power conversion operation, for example, switching frequencies are varied in order to control output voltages. In such a switching power supply apparatus, a resonant inductor Lr and a resonant capacitor Cr compose a first LC resonant circuit and an excitation inductor Lm of a primary winding of a transformer, the resonant inductor Lr, and the resonant capacitor Cr compose a second LC resonant circuit.
Provided that the switching frequency is denoted by fs, the resonant frequency of the first LC resonant circuit is denoted by fr, and the resonant frequency of the second LC resonant circuit is denoted by fm, a relationship fm<fr<fs is established during the normal operation. The switching frequency fs is increased to decrease the output voltage in light load while the switching frequency fs is decreased to increase the output voltage in heavy load. Under the above frequency magnitude relationship, the switching power supply apparatus operates in a “current delay phase” in which the current flowing through the primary winding of the transformer is delayed in phase from the voltage applied to the primary winding thereof.
However, the switching frequency fs is decreased with the increasing load and the switching power supply apparatus enters a state in which a resonance condition is not met (an “out-of-resonance” state) if fs<fm<fr. In other words, the relationship in which the switching frequency fs is lower than the resonant frequency corresponds to a state in which the transformer appears as a capacitive impedance from the primary side circuit and the current waveform is advanced in phase from the waveform of the voltage applied to the primary winding of the transformer. In this case, a period in which a low-side switching element and a high-side switching element are simultaneously turned on (a so-called arm short circuit) occurs and there is a problem in that excessive current flows through the two switching elements to cause heavy loss.
Specifically, in the above state in which the current waveform is advanced in phase from the voltage waveform, after the low-side switching element is turned off, the high-side switching element is turned on with a dead time sandwiched between the turn-off of the low-side switching element and the turn-on of the high-side switching element. However, when the high-side switching element is turned on in a state in which the polarity of the current flowing through the low-side switching element has already been reversed (the current flows through a body diode of the low-side switching element), the high-side switching element conducts in a state in which the body diode of the low-side switching element conducts due to delay in blocking caused by reverse recovery characteristics of the body diode to cause the arm short circuit.
Since it is not possible to perform zero voltage switching (ZVS) (soft switching) in the state in which the current waveform is advanced in phase from the voltage waveform, there is a problem in that the switching loss is increased.
The switching power supply apparatuses in which the “out-of-resonance” state is prevented are disclosed in Japanese Unexamined Patent Application Publication No. 9-308243 and Japanese Unexamined Patent Application Publication No. 11-332232. In the above switching power supply apparatuses, the switching elements are controlled so that current flowing through the transformer or current flowing through the switching elements is detected and the value of the detected current is compared with a predetermined value to prevent the “out-of-resonance” state.
Since it is necessary to constantly monitor the current flowing through the transformer or the current flowing through the switching elements in the switching power supply apparatuses disclosed in Japanese Unexamined Patent Application Publication No. 9-308243 and Japanese Unexamined Patent Application Publication No. 11-332232, there are problems in that not only the loss is increased but also the power supply apparatuses are increased in size due to the circuit configurations.
Although a method of setting the switching frequency fs to a higher value in advance in order to avoid relationship fs<fm is available, this method does not support a case in which the input voltage is low and the output voltage is high or a case in which there is great variability among the transformer and electronic components.